One problem which often occurs when transmitting radio signals over a channel is that a transmitted signal is subjected to multipath propagation and noise. In, for instance, mobile telephony, there is the additional problem that the transmission properties of the channel vary as a result of a mutual shift in the positions of transmitter and receiver. These problems have been solved in time-shared, digital radio transmission systems, in that the signal sequences transmitted in a time slot are made to include a synchronizing sequence and a data sequence. The synchronizing sequence is known to the receiver, and the receiver is able to estimate the transmission properties of the channel, (a channel estimate), with the aid of this sequence. This channel estimate enables the receiver to estimate the symbols of the data sequence which contains the information to be transmitted.
However, in certain instances, it is found insufficient to effect solely one channel estimation with each time slot. In the case of long time slots, in the order of several milliseconds, there is time for the transmitter and the receiver to change their respective positions quite considerably in the duration of the time slot. This means that the transmission properties of the channel can change substantially within the duration of the time slot, so that the estimation of the transmitted symbols carried out by the receiver will be deficient and the transmitted information will contain interferences or disturbances. A radio receiver in which these interferences are partially avoided is described in an article in IEEE Transactions On Information Theory, January 1973, Pages 120-124, F. R. Magee, Jr. and J. G. Proakis: "Adaptive Maximum Likelihood Sequence Estimation for Digital Signaling in the Presence of Intersymbol Interference". The article describes an equalizer which includes a viterbi analyzer having an adaptation filter as a channel estimation circuit. The viterbi analyzer estimates a received symbol, and the estimated symbol is compared with the received signal in the adaption filter. The coefficients of the adaptation filter are adjusted with the aid of an error signal obtained in the comparison , and these coefficients are in turn utilized in the viterbi analyzer to estimate new symbols. The equalizer can utilize as a starting value the channel estimate obtained with the aid of the synchronizing sequence, this channel estimate being updated for each new symbol in the data sequence. The equalizer described in the article overcomes partially those problems which occur with long time slots, but has the drawback of being relatively slow and of providing less satisfactory symbol estimation when channel conditions vary rapidly. This is because the adaptation filter is set with the aid of the symbols estimated in the viterbi analyzer, these symbols being time delayed in relation to the received signals.